1. Field of the Invention
The present invention relates to a data communication system that is provided with one main station and one or a plurality of substations, and more particularly to a data communication system for broadcast communications from the main station to the substations.
2. Description of Related Art
Before now, data communication systems having one main station and one or a plurality of substations were known. These data communication systems are systems for broadcast communication from a main station to the substations. Broadcast communication is communication wherein data addressed to one or a plurality of substations are transmitted from one main station. With broadcast communication, the same data can be transmitted to one or a plurality of substations substantially at the same time. In the case of effecting broadcast communication, the main station queries the substations as to whether the data were received normally once a prescribed quantity of data is transmitted. This query operation is generally called polling. Polling methods are disclosed in “Information communications protocol”, Tadao Saito and Mitsuhiro Ishizaka, pp. 73, Ohm Co., for example.
In polling by a conventional data communication system, the main station queries each substation individually as to whether the data were received normally. For this reason, the main station must perform the query operation for the same times as the number of substation that received the data. Consequently, when a large number of substations received data, the query operation is performed many times and the data communication becomes inefficient.